BRD4 is emerging as an important player in cancer: in the etiology of cancer, metastasis, and epithelial-mesenchymal transition. Indeed, inhibition of BRD4 dramatically reduces aggressive AML, promotes regression of NUT midline carcinoma, and is immuno-modulatory, making it an attractive therapeutic target. Additionally, BRD4 regulates cell cycle progression, transcription elongation and viral infection. The wide range of BRD4 activities makes understanding its molecular mechanism(s) of action of interest to a broad community. Despite the wide-ranging interest in BRD4, there is relatively little understanding of its function. By binding to acetylated chromatin through its bromodomains, BRD4 recruits a variety of transcription factors to regions of active chromatin, including super-enhancers and typical enhancers and thus plays a passive role in transcriptional regulation. The goals of our studies is to determine whether BRD4 plays an active role in regulating transcription. Indeed, we have discovered that BRD4 is an atypical kinase that directly regulates eukaryotic transcription, providing new insight(s) into BRD4 activity and function. The goal of our studies is to further characterize the mechanisms by which BRD4 regulates transcription through its biophysical characterization. As an atypical kinase, BRD4 does not have a discrete active site. Rather its activity is constituted by a series of kinase domains, dispersed throughout the molecule. Mutation of one, or even a few, of the kinase domains does not inhibit its kinase activity. However, deletion of the B domain of BRD4 results in a kinase-deficient mutant, which is unable to phosphorylate its substrates either in vivo. Surprisingly. activity of the mutant can be restored under certain conditions of purification, indicating that its activity is conformationally regulated. To gain further insight into the conformation of BRD4, analytical unltracentrifugation was performed that revealed that BRD4 exists in an extended conformation. It also documented BRD4 binding to the RNA polymerase CTD. Although the kinase activity of BRD4 is distributed throughout the molecule, NMR studies have demonstrated that the BRD4 ET domain is the binding site of its CTD substrate. Importantly, other peptides that bind to the ET domain are also phosphorylated. Together these findings provide greater insights into the structure/functional relationship of BRD4.